Prisms won’t cause a fire, but I discovered that crystal balls can.
A friend gave my little sister a crystal ball. It was a hunk of glass about the size of a small melon. For a while it sat on the table in the kitchen, then we moved it outside for a while. Quite a while later my mom asked me if I had been playing with fire or something hot on the kitchen table and I said no. She didn’t really believe me, but I knew I hadn’t.
Flash forward to maybe a few weeks later. I’m sitting on the porch (where we had put the crystal ball) and I smell a faint whiff of smoke. Looking around, I notice that there is a very small amount of smoke drifting up from spot on the table. The smoke is coming from one end of a small arc of blackened wood a short distance away from the crystal ball.
Gears start turning in my mind and I begin to put things together. The crystal ball was acting as a magnifying glass and burning a track across the wood! Then I realize that I’ve seen a track very similar somewhere else: on the kitchen table. I go inside and look and sure enough, its the same arc, just longer and deeper.
After that we always kept the crystal ball out of the sun.
Do you still have access to those tables? I’ll bet you that if you took a close look at the tracks, ther’d be two points along them where the burns were narrower and deeper, and that further along the tracks, the burns got wider and faded out. A ball will have a significantly aberrated focus, but there would still be some optimum distance for it to focus at, and that distance would most likely occur at two points along the track.
Now, myself, I’d make it a point to always keep the ball in the sun after that, and just take care that whatever it was sitting on was something OK to burn. But that’s me.
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